The present invention relates to high speed drilling systems for precision drilling of workpieces such as printed circuit boards and the like, and more particularly to systems for drilling very small diameter holes in such workpieces at high speed.
Printed circuit boards are in universal use today to mount and interconnect electrical components forming electrical circuits. Typically, the leads of the components are inserted through holes drilled in the boards to form predetermined hole patterns. Various systems are known in the art for automated drilling of the holes in printed circuit boards. Examples of such systems and components thereof are described in U.S. Pat. Nos. 4,088,417; 3,578,827 and 3,973,863. Such systems typically employ multiple spindles, and are numerically controlled to drill the workpieces in accordance with a predetermined drilling sequence. The systems typically employ automatic tool-changing devices to allow the drill tool to be rapidly changed during the drilling sequence. The workpiece is mounted on a table supported for X-Y translational movement to accurately position the workpiece in relation to the drill spindles. Typically, the work table is supported by a plurality of air bearings on a granite beam, and a lead screw mechanism is provided to allow the capability of translation along one of the X-Y axes. The granite beam is in turn typically supported above a granite base by a plurality of air bearings and another lead screw mechanism provides the capability of translation along the other of the X-Y axes.
Printed circuit boards used today may employ holes of very small diameters, e.g., as small as four thousandths of an inch, with as many as forty thousand holes per board, to accommodate high component densities on the board. There are a number of problems in drilling such small holes with high speed and the required precision.
One of the most detrimental problems for small hole drilling is the Z axis vibration during drilling, which results in drilling tool breakage. Printed circuit board drilling machines are typically multi-station devices to increase the throughput, and relatively large moving tables are required to accommodate all work stations. To meet high speed operating requirements, the work table supporting the workpieces are made as lightweight as possible. Because the conventional work table is constructed from cast iron, the table is generally quite thin to reduce the weight. In conventional systems, these tables are usually supported by the air bearings at four or six points. This configuration results in a "drum" effect when the pressure feet for the respective tool spindles hit the table supported workpiece to hold the workpiece down as the spindles are lowered for drilling operation. The impact of each pressure foot causes vibrations in the table creating motion in the direction of the advancing bit, consequently dramatically increasing the chip load on the drill bit. This condition is one of the main causes of small drill bit breakage. Work table oscillations also result from the acceleration and deceleration of the work table to rapidly position the workpieces.
Conventional spindles presently used in P.C.B. drilling systems rotate the drill as it is gripped by one of many types of chucking devices. One type of chuck is the centrifugal chuck which apply gripping forces on the drill bit caused by the centrifugal forces set up by the high speed rotation of the spindle and bit. There are some disadvantages to these conventional chuck types, including insufficient bit clamping force at lower speeds, lack of concentricity of the bit and the spindle axis, control of the runout of the bit, and control of the depth of insertion of the bit. Moreover, in conventional systems, the entire spindle, including the heavy spindle body, is lowered and raised to accomplish the drilling, thereby tending to set up relatively high reaction forces and vibration during the rapid acceleration and deceleration of the spindle required for high speed drilling operations.
It would therefore represent an advance in the art to provide a drilling system capable of high speed drilling of small diameter (on the order of 0.003 to 0.06 inches in diameter) holes in workpieces such as printed circuit boards and substrates.